The invention relates to a filter to be placed in the blood system and in particular in the course of a vein for retaining blood clots.
Filters of this type are described for example in U.S. Pat. No. 4,088,553.
Generally, these filters are in the form of a small frustoconical basket which attaches to the interior of a vein downstream of the path which is desired to be filtered; this is generally the vena cava arriving at the heart. It is thus possible to stop any blood clots which can form and, in particular causes embolisms, before such clots enter the heart.
Usually these filters are of two main types.
The first type noted is filters produced from metal wires assembled together by welding or crimping. These filters are small and flexible. Once bent, they are not bulky and are well-suited to insertion by routes which are difficult and have pronounced anatomical curvatures.
Furthermore, there are noted filters produced from plates generally enabling the filtering portion to be produced in one part which is cut out and shaped. These filters have a large bearing surface in contact with the vein, thus reducing trauma and eliminating the risks of perforation.
However, these two types of filters have various disadvantages.
With respect to the filters produced with wires, the flexibility of the wires causes two considerable problems:
firstly it prevents consistent bearing forces from being applied to the vein wall, the filter is thus not held well and can migrate;
secondly, the wire is small, and the bearing surface in contact with the vein is then restricted which can give rise to trauma and perforations.
Furthermore, there is a risk of the wires becoming tangled at the moment when the device is fitted, thus reducing the filtering capacity of the filters.
The filters produced from plates have a high degree of rigidity in their confined fitting position which renders them awkward to fit via routes which are initially sinuous. These filters are considerably bulky owing to the fact that the surfaces thereof which do not permit very fine insertion devices to be used increase trauma at the point of puncture.
Furthermore, filters formed from a conical portion provided with filtering components are generally difficult to position correctly. Generally, in order to introduce a filter of this type into the blood vessel, it is pushed there by means of a tube known as a catheter which passes through the said vessel and of which the diameter is less than that of the latter. When the filter arrives at the end of the insertion tube, it is released into the vessel and the expansion of its fingers, which are frequently provided with hooks, anchors it in place.
In practice a "release" of this type is very awkward to control. In a certain number of cases it has ensued that the basket-type filter in fact occupies a position other than the most favourable position inside the vessel, i.e., a position in which the axis of the filter is substantially parallel with the axis of the vessel.